1. Field of the Invention
The present invention relates to optical sensors for monitoring the level of fluid in containers. In the operation of various kinds of motor driven equipment, it is often necessary to monitor the liquid levels in a container. In readily accessable or visible containers, such as most automotive equipment, it is possible to use dip sticks and floating indicators to measure the levels of liquids in the container. In less accessable situations, such as where there is a need to measure the fuel level in fuel tanks of operating airplanes or in other closed container situations, a variety of optical level sensing devices have been used to detect liquid level problems and to electronically or otherwise alert the operator of the equipment to liquid level problems
One type of prior art sensor uses a light source and a detector to determine whether light that enters a prism inserted into a container (e.g. a fuel tank) passes through the prism into the surrounding liquid to indicate an adequate liquid level or is reflected within the prism to indicate the absence of a liquid. In such devices, a visual or electrical signal is generated to indicate the absence of liquid A problem with such existing devices is that there is no way to instantly detect the failure of the light source or detector, which failure could mimic the indication of a satisfactory condition. This could result in the absence of an alerting signal when, in fact, there is a dangerously low liquid level Obviously, generating a false reading that indicates sufficient liquid levels when levels are not sufficient could have disasterous consequences when used in moving aircraft or ground vehicles
The present system improves upon the prior art by using paired systems of light sources and sensors (e.g. light emitting diodes and light sensitive transistors). In the present system, particular photodetectors are located in positions to detect and respond to discrete light sources so as to provide output that can be converted to panel gauge readings for use in, for example, an airplane. Such panel readings can, in addition, be adapted to enable the operator to either visually or by electronic means detect a non-operational light source.
The ability of adjacently located photodetectors to sense only one of two light sources both of which are totally internally reflected by a conical prism was heretofore unexpected. Previously it was thought that the simultaneous total internal reflection of two light sources directed at different surfaces of a conical prism would be so diffuse as to cause two light detectors that were in physical proximity to each other to both respond to reflected or scattered rays from both sources. It was found, however, that a single photodetector responds only to the light beam from its paired light source.
2. Description of the Prior Art
To meet the need for a reliable level sensor for use where level detection is critical to the performance of equipment, a number of optical sensors using the principles of refraction and total reflection have been developed. These devices work on the principle, known as Snell's Law, whereby light rays are refracted when they move from one material to another. The ratio of the velocity of light in a particular material to the velocity of light in a vaccum is called the index of refraction "n" for that material. The relationship between the incident and refracted rays of light passing through an interface between the materials is given by: EQU n sin i=n' sin i'
where n is the index of refraction of the first medium and n' is the index of refraction of the second medium; "i" is the angle to the normal of the incident ray and "i'" is the angle of refraction (angle between the refracted ray and the normal). Total internal reflection of the incident light occurs when the refracted light is bent away from the normal so that it crosses the interface between the two materials (i'=90). Total internal reflection occurs whenever; EQU i&gt;sin.sup.-1 (n')
Examples of the use of optical liquid sensors occur in the patents mentioned below.
U.S. Pat. No. 4,670,660 teaches a self sufficient visual liquid level sensor that uses light refraction measurements from an illuminated prismatic member. Such a visual sensor is not adapted for use in closed containers contemplated by the present inventions, such as fuel tanks of airplanes
U.S. Pat. No. 4,286,464, by Tauber et al, uses a plurality of prisms and electronic detectors arranged in a linear array The "bulls-eye" type of prism detectors are able to detect liquid levels surrounding a prism where the prism is not totally immersed in a liquid. The prisms and coordinated detectors, which are each illuminated by a small light source, are arranged vertically so that in use each one will sense a different level of liquid. As the array of prismatic detectors are sequentially illuminated by light sources, they emit a visual output in the form of a short pulse of light. By electronically counting the number of pulses detected in any single scan and converting these pulses to an analog or digital panel reading (e.g. for use in aircraft), calculated as a function of the empty height of the container, a composite fluid height is determined at a particular instant.
U.S. Pat. No. 4,450,722, by Keyes et al, discloses a liquid level detector which comprises a liquid column in which different color lights detect the presence or absence of water in the column. A dual detector assembly of color detectors alerts an operator to the presence or absence of a water condition.
U.S. Pat. No. 4,246,489, by Yoshida et al, includes a rectangular prism refractor for transmitting a particular wavelength through the prism. The invention utilizes a light emitting element and a photo electric conversion element adjacent to the face of the prism.
U.S. Pat. No. 4,354,180, by Harding teaches a sensor comprising a transparent body having a means to shine a light beam on the surface area of the body and through the body into a liquid or dry medium. The innovation is a means for intercepting a portion of the beam and reflecting the intercepted portion of the beam to an area where it is detected by an alarm activating means. Thus, a failure of a light source can be detected by means of optical electronics, when used in a circuit as part of electronic components.
U.S. Pat. No. 4,366,384 teaches a detector for detecting air bubbles within a conduit. It consists of a conduit for the passage of fluid in which the conduit has an inner wall that is reflective. A light source is positioned beside the conduit. There is a light detecting means for sensing direct light passing through the fluid in the conduit and a second light detecting means for receiving light reflected by the inner wall of the conduit. Where the difference in the direct light and reflected light detected is greater than a reference level, the presence of air bubbles is indicated in the circuit.
U.S. Pat. No. 3,683,196 teaches a differential fluid level detector for sensing the presence of a fluid medium at a first and second medium level. At least one light sensor is placed in optical communication with the pre-determined light reflection path. A light emitting means emits a light beam through a light refracting means such as a prism. The light refracting means includes a first and second light refracting body portion.
U.S. Pat. No. 4,242,590 teaches a single light source prism detector with a photo electric sensor at the nonsensing ends of the detector. A circuit connected to the photo electric detector detects reflected light and has a hysteresis effect for avoiding extreme fluctuation in responses to fluid slopping "to and fro" in the container.
U.S. Pat. No. 3,882,887 teaches an optical level sensing system having a light emitting diode that produces pulsating light that is transmitted through a prism. A sensing device indicates changes in the reflection of the light to show differences in the fluid level around the prism.
The previously mentioned devices lack the dual light source and dual sensor system, combined with a single prism, for detection of a liquid level as found in the present invention.